1. Technical Field
The present disclosure relates to pedicle screws and, more particularly, to a polyaxial screw assembly.
2. Background of Related Art
The adult human spinal column has more than twenty discrete bones sequentially coupled to one another by a tri-joint complex consisting of an anterior disc and the two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The bones of the spinal column are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine is the top of the spine and includes the first seven vertebrae from the base of the skull. The next twelve bones are the thoracic vertebrae. Thereafter, the next five bones form the lumbar vertebrae. Connecting to the lumbar, the sacral bones are at the base of the spine and include the coccyx.
The spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies. Spinal pathologies either limit the range of motion of the spine or threaten the critical elements of the nervous system housed within the spinal column. There are many systems that immobilize the spine by implanting artificial assemblies in or on the spinal column. The implantable assemblies are classified as anterior, posterior, or lateral implants. As the name of the classification suggests, lateral and anterior implants are coupled to the anterior portion of the spine. Posterior implants generally comprise rod assemblies that are attached to the spinal column by either 1) hooks coupled to the lamina or the transverse processes, or 2) by screws inserted through the pedicles.
The screws are implanted through the posterior lateral surfaces of the laminae, through the pedicles, and into their respective vertebral bodies. The screws have upper portions that are coupling elements for receiving and securing an elongate rod. The elongate rod extends along the axis of the spine and is coupled to the screws via the coupling elements. The rigidity of the elongated rod aligns the spine in a more desired shape.
Inserting screws along a misaligned curvature of the spine, while exactly positioning the coupling elements, can be difficult. The coupling elements must be placed in a way that a rod can be passed therethrough without damaging the screws. As a result, the surgeon must be careful when trying to properly align fixed screws, which increases the operating time and leads to an increased probability of a complication.
The art contains screws that permit a limited amount of freedom with respect to angulation of the coupling element. These assemblies, however, are generally complex, unreliable, and lack durability.